514 Forbes — The Microscope in Geology. 



Another section of a crystalline slag, produced in silver smelting, 

 is given for the sake of comparison with the structure of erup- 

 tive rocks. In formation it is so nearly identical with what is seen 

 in sections of more felspathic basaltic rocks, the mass of which 

 consists of a framework of interlaced crystals of labradorite with 

 the interstices filled up with the other mineral constituents confusedly 

 crystallised, that this section might easily be mistaken for such. 

 The Eowley rag, when fused and very slowly cooled, presents a 

 similar appearance ; and, in general, the structure of crystalline 

 slags presents many features in common with that of ordinary 

 eruptive rocks. 



Before proceeding to the next class of rocks, the discovery by 

 Sorby of the numerous minute fluid cavities in the quartz of granites 

 should be alluded to, as proving the great value of the microscope 

 in the study of these rocks. The result of this gentleman's re- 

 searches ^ proves that granites have solidified at a heat far below the 

 fusing points of their constituent minerals, and at sxxch a pressure as 

 to enable it to entangle and retain a small amount (|- to ^ per cent.) 

 of aqueous vapour, which naturally must have been present during 

 its liquefaction. The presence of these fluid cavities in the quartz 

 of granite was immediately blazoned forth as proof positive of the 

 non-igneous origin of granite ; whereas, if Mr. Sorby's memoir had 

 actually been read, it would have been seen that he had found fluid 

 cavities, perfectly identical with those in granite, not only in the 

 quartz of volcanic rocks, but also in the felspar and nepheline 

 ejected from the crater of Vesuvius, and that the presence of fluid, 

 vapour, gas, and stone cavities, are common both to the volcanic 

 quartz-trachytes and to the oldest granites ; and the inference drawn 

 by Mr. Sorby, from the results of his researches, is that both these 

 rocks were formed by identical agencies. He therefore classes 

 them together under one head as rocks of similar origin.'^ 



II. Secondary ob Sedimentary Eocks. 



The rocks pertaining to this class are all, directly or indirectly, 

 formed from the breaking up, or debris, of previously existing 

 rock, and, for that reason, might, as before -mentioned, not inappro- 

 priately be termed derivate rocks. When found in the normal state 

 of sedimentary deposition, they may be conveniently subdivided 

 into — 



1. Eocks formed of the immediate products of the breaking-up of 

 eruptive rocks. 



1 Quart. Jour. Geol. Soc, toI. xiv. pp. 453-500. 



2 These researches tend to confirm the theory of the igneous origin of granite and 

 eruptive rocks in general. It must not be forgotten that by igneous action, as used 

 by the Plutonist, was always understood the action of heat as developed in volcanoes 

 (the study of which was the basis of the theory itself), in which the agency of 

 water was always recognized. Nearly half a century ago, Scrope not only insisted 

 on the important part played by water in volcanic action, but specially pointed out 

 the difference between such volcanic fusion and ordinary melting. The term hydro- 

 igneous action might not be inappropriate for such, but hydro-thermalism does not at 

 all express what is intended. The idea of a true dry fusion in nature exists only in 

 the brains of the ultra-Neptunist or lukewarm hydro thermalist. 



